Allow users to group button blocks in course page

[gikaragia]

Is your feature request related to a problem? Please describe

Currently all button blocks in the course page, are single buttons and cannot be grouped in a similar manner as with core/buttons block or the lesson page.

Describe the solution you'd like

The ideal solution solution would be to be able to add the buttons inside core/buttons. This would give a lot of flexibility to the users and leverage functionality provided by Gutenberg. We could also add a variation of core/buttons which adds all Sensei buttons as inner blocks by default for visibility purposes. Currently there is no direct way to modify the allowed blocks so we should probably consider to do one of the following:

• Create a new block for each button that would set core/buttons as a parent. This way the block can be added although it is not specified in allowedBlocks.
• Create a button block variation for each button. Variations would be considered as a core/button block and it would be possible to add them into core/buttons.

Describe alternatives you've considered

• Create a custom grouping block. Rejected as it seems that it would be best if we can leverage core Gutenberg functionality.